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Oct. 4, 2010 -- Getting a flu shot during pregnancy may be one of the most effective ways to protect infants from getting the flu.

A new study confirms that babies whose mothers received the influenza vaccine while pregnant were much less likely to become ill with the flu or be hospitalized for respiratory illness in their first six months of life.

In the U.S., children under 6 months of age are more likely to be hospitalized due to complications of the flu than any other age group of children. Infants are especially vulnerable to infectious diseases like influenza during the first months of life, but their immune systems are not developed enough to respond to certain vaccines, like the flu vaccine.

"Although influenza vaccination is recommended for pregnant women to reduce their risk of influenza complications, these findings provide support for the added benefit of protecting infants from influenza virus infection up to six months, the period when infants are not eligible for influenza vaccination but are at highest risk of severe influenza illness," write researcher Angelia A. Eick, PhD, of the Johns Hopkins Bloomberg School of Public Health, and colleagues in the Archives of Pediatrics & Adolescent Medicine.

"These findings are particularly relevant with the emergence of 2009 pandemic influenza A (H1N1) virus, which had a substantial impact on pregnant women and high hospitalization rates among young infants," they write.

Mom's Flu Vaccine Protects Infant

In the study, researchers followed 1,169 mothers who lived on Navajo and White Mountain Apache Indian reservations and their infants who were delivered during one of three flu seasons. The mothers completed a questionnaire about their infant's health at the end of the flu season; 1,160 mother-infant pairs gave blood samples that were analyzed to confirm the presence of the flu virus.

The results showed 17% of the infants were hospitalized for a flu-like illness, and an additional 36% were treated on an outpatient basis for the flu during the first six months of life.

Among confirmed cases of influenza, infants whose mothers had the flu shot had a 41% lower risk of becoming infected with influenza and 39% lower risk of hospitalization due to flu-like illness.

The study also showed that infants whose mothers had the flu vaccine had higher levels of protective antibodies against the flu at birth and at 2 to 3 months of age than those whose mothers did not get the flu shot.

Influenza Vaccination During Pregnancy Recommended

The U.S. Advisory Committee on Immunization Practices (ACIP) has recommended the use of the influenza vaccine during pregnancy since 1997, but researchers say there has been little increase in use from 1997 to 2009.

In an editorial that accompanies the study, Justin R. Ortiz, MD, and Kathleen M. Neuzil, MD, MPH, of PATH and the University of Washington, Seattle, point to previous studies that suggest many pregnant women believe influenza infection is not serious and have misconceptions about the safety of the flu vaccine during pregnancy.

They say the additional benefits of maternal flu vaccination to the newborn should catalyze efforts to improve vaccination rates in countries with existing maternal immunization recommendations and encourage the adoption of such guidelines in countries without them.

"Maternal influenza vaccination targets two high-risk groups with one vaccine dose -- we can't afford not to act," write the editorialists.